1. Field of the Invention
The present invention relates in general to loudspeakers and in particular to a diaphragm for a loudspeaker that significantly improves the quality of sound and the usable life of the loudspeaker.
2. Description of Related Art Including Information Disclosed Under 37CFR 1.97and 1.98
A typical loudspeaker transducer 10 , as shown in FIG. 1, has a cone 12 and/or dome 14 , diaphragm that is driven by a voice coil 16 that is immersed in a strong magnetic field. The voice coil 16 is electrically connected to an amplifier and, when in operation, the voice coil 16 moves back and forth in response to the electromagnetic forces on the coil caused by the current in the coil, generated by the amplifier, and the stationary magnetic field. The cone 12 and voice coil 16 assembly is typically suspended by a "spider" 18 and a "surround" 13 , a flexible connector to frame 20. This suspension system allows the cone and coil assembly to move as a finite excursion piston over a limited frequency range. Like all mechanical structures, cones and domes have natural modes or "Mode peaks" commonly called "cone break-up". The frequency at which these modes occur is largely determined by the stiffness, density, and dimensions of the diaphragm, and the amplitude of these modes is largely determined by internal damping of the diaphragm material. These mode peaks are a significant source of audible coloration and, as a result, degrade the performance of the loudspeaker system.
Designers have tended to take two paths to solve the cone break-up problem. For small diaphragms such as those found in dome tweeters, aluminum and titanium are commonly used. In these applications, the dome dimensions can be manipulated such that the first natural modes of the dome are above the frequency range of human hearing. FIG. 2 shows the frequency response of a typical 1" titanium dome tweeter (note the large mode peak 22 at 25 kHz). The amplitude of these modes is usually very high because metals have very little internal damping. For diaphragms larger than approximately 1", the dome modes fall into the audible range. These modes are plainly audible as coloration because of the high amplitude of the modes. FIG. 3 shows the frequency response of a typical 3" titanium dome mid-range speaker (note several large peaks 24, 26, and 28 at 11 kHz, 16 KHz, and 18 kHz).
For larger diaphragms, softer materials such as polymers or papers are commonly used. These materials have several natural modes in the band in which they operate. However, the internal damping of these materials is high enough so that most of these modes do not cause audible coloration. The remaining modes are either compensated for in other parts of the loudspeaker system design, resulting in increased costs, or are not addressed at all, resulting in lower performance. FIG. 4 shows the frequency response of a typical 5" woofer with a polypropylene cone (note the large mode peaks 30 and 32 at 4 kHz and 5 kHz).
Many metal diaphragms feature a thin anodized layer. Typically, the metal is anodized to provide a specific color to the visible surface, or to protect the metal from sunlight, humidity, or moisture.
Ceramic materials such as alumina or magnesia offer significantly higher stiffness numbers and slightly better internal losses than typical metals such as titanium or aluminum. As a result, the natural modes of diaphragms made of these materials are moved higher in frequency and reduced in amplitude and, thus, reduce audible coloration. For instance, FIG. 14 shows the frequency response of a 5" woofer with a ceramic metal matrix cone of the present invention. Note that the mode peaks 34 and 36 occur at approximately 6.5 kHz and 8.5 kHz. Compare FIG. 14 to FIG. 4. The mode peaks 34 and 36 have moved to a significantly higher frequency than mode peaks 30 and 32 in FIG. 4. This frequency extension allows a more simple and economical roll-off circuit, well known in the art, to be constructed to eliminate the unwanted frequencies.
Table I shows the important structural parameters for several materials. Unfortunately, pure ceramics are very brittle and are prone to shattering when used as loudspeaker diaphragms. Additionally, making diaphragms of appropriate dimensions can be very expensive. As a result, pure ceramic loudspeaker diaphragms have not become common.
TABLE I PROPERTIES OF DIAPHRAGM MATERIALS Internal Young's Modulus Speed of Loss Material (Stiffness) Density Sound (damping) Paper 4 .times. 10.sup.9 Pa 0.4 g/cm.sup.3 1000 m/sec 0.06 Polypropylene 1.5 .times. 10.sup.9 Pa 0.9 g/cm.sup.3 1300 m/sec 0.08 Titanium 110 .times. 10.sup.9 Pa 4.5 g/cm.sup.3 4900 m/sec 0.003 Aluminum 70 .times. 10.sup.9 Pa 2.7 g/cm.sup.3 5100 m/sec 0.003 Alumina 340 .times. 10.sup.9 Pa 3.8 g/cm.sup.3 9400 m/sec 0.004